Most nights, from around 7 till midnight, Sydney Jade is on TikTok, the smartphone app of the moment. The platinum blond teenager films herself singing show tunes, doing jumping jacks, and joking around with store clerks at a Walmart not far from her home in Oklahoma. Her short music videos and livestreams are popular—Jade has 284,000 followers, some of whom periodically send her virtual gifts, like 99¢ Rainbow Puke stickers.
Jade’s parents resisted TikTok at first. They hadn’t heard of the app and, Jade says, “didn’t like the idea of strangers watching me sing alone in front of the ...